Membrane Permeability Flashcards
What molecules can pass freely across the plasma membrane?
Small uncharged polar molecules (water, urea, glycerol)
Hydrophobic molecules and gases (o2, CO2, n2, benzene)
If Gibbs free energy △G is positive the diffusion is _________
Active - change in free energy
If Gibbs free energy △G is negative the diffusion is _________
Passive
What is active transport?
Allows movement of ions/molecules against their concentration or electrical gradient (unfavourable)
Energy required indirectly or directly from ATP hydrolysis
Which cells have high energy expenditure on active transport?
Nerves
Muscle
Erythrocytes
Enterocytes
What are the general intracellular and extracellular Na concentrations?
Outside 145 mM
Inside 12 mM
What are the general intracellular and extracellular K concentrations?
Outside 4 mM
Inside 155 mM
What are the general intracellular and extracellular Cl concentrations?
Outside 123 mM
Inside 4 mM
What are the general intracellular and extracellular Ca concentrations?
Outside 1-2 mM
Inside 100 nM
What is uniport?
Only one type of molecule transported at a time
What is cotransport?
More than one ion/molecules transported at a time
What is symport?
Cotransport in the same direction
E.g. Na/Glucose cotransported in small intestine and kidney
What is antiport?
Cotransport in opposite directions
E.g. NCX.
What is the function of the Na/K ATPase?
Active transporter present in every cell
Transports 3Na out of cell and 2K into cell per ATP molecule hydrolysed
Acts to maintain cellular concentrations of Na and K (form the gradients)
What channels are responsible for resting membrane potential (~-70mV) ?
K diffusion through K+ channels
What is the main function of the K/Na ATPase?
Form sodium and potassium gradients which are necessary for electrical excitability
Also derived secondary active transport (cotransport)
How does the Na/k ATPase regulate ph?
Sodium gradient generated by Na/k ATPase is used to exchange H+ ions out of the cell (1:1 exchange)
What is high intracellular toxic to cells?
Causes precipitation of phosphate (ossification) and Pi can’t be recycled
What is the role of mitochondrial Ca uniporters?
At high cytoplasmic calcium, operate as a buffer to take calcium into mitochondria (facilitated diffusion), induces caspase pathway which induces apoptosis of the cell (cell death at high [Ca2+]
In control of pH which two exchangers are important in lowering acid (H+ leaves)?
- Na/H exchanger (NHX) - one Na in, one H out
- Na dependent Cl-/HCO3- exchanger - one Na in , 1 H out, 1 HCO3 in, 1 Cl out
What does electroneutral mean?
The number of positive/negative charges leaving is equal to the the charge entering
E.g. Na/H exchanger
What is the Na/H exchanger activated and inhibited by ?
Activated by growth factors
Inhibited by amiloride (K+ sparing diuretic)
In control of pH which exchanger is important in lowering base inside cell (HCO3- leaves)?
- Cl/HCO3- anion exchanger - one HCO3- out, 1 Cl in
Important in peripheral circulation
What molecules are important in cell volume regulation?
Water follows osmotically active ions such as Na, K, Cl (6 molecules of water move across membrane per one ion) and organic osmolytes (such as amino acids) into/ out of cell causes shrinking or swelling
What mechanisms resist cell swelling (remove water from cells)?
Conductive systems - facilitated diffusion of K and Cl out of cell (no change in electronegativity), water follows
Cotransport systems - H/K exchanger and cl/HCO3- exchanger lead to H + HCO3 = H2CO3 -> CO2 + H2O leave cell
- K/Cl cotransport out of cell, water follows
What mechanisms resist cell shrinking (add water to cells)?
Conductive systems - facilitated influx of Na or Ca, water follows
Cotransport systems - influx of H2O and CO2 formation of H2CO3 which forms H (leaves cell, Na enters) and HCO3- (leaves cell, cl enters)
- Na/Cl cotransport into cell, water follows
In an average human (70kg), how much of the body is water?
60% water
60% of 70 = 42 L water
How much of the body’s water is extracellular?
1/3 of the body’s water is extracellular (2/3 intracellular)
So in a 70kg person, 42 L of water - 14 L extracellular
- 28 L intracellular
Of the extracellular water, how much is interstitial and how much is in plasma?
In a 70kg individual, 14L extracellular water
11L interstitial water
3L plasma
What is the circulating blood volume of a 70kg individual?
5L circulating blood volume
- 3L plasma
- 2L red cells
What are the consequences of an increase in the permeability of blood capillaries to plasma proteins?
Plasma proteins leave capillaries which lowers oncotic pressure in the capillaries, enters interstitial space, increases hydrostatic pressure, water follows causes odema
What are the glucose transporters on the intestinal epithelial cells?
On the lumen face of the epithelial cells - Na/glucose symporter (uses Na gradient, to transport glucose from intestine into cells)
Between epithelial cell and capillary, GLUT2 which transports glucose out of cells into capillaries
Where in the body are Na/glucose symporters found?
Intestinal cells
Kidney epithelial cells
What glucose transporter is present in adipose, brain, liver and skeletal cells?
Have an insulin receptor that upon insulin binding, causes insertion of glucose channels into the plasma membrane (via exocytosis of vesicles containing channels) allows glucose influx into cells
How is glucose that enters cells prevented from leaving?
Converted into either glycogen (storage) or G6P (respiration - glycolysis)
What other organic molecules use Na gradient for transport unfavourably up gradient?
Vitamin C
Amino acids
